The present invention relates to oil recovery techniques in which the recovery of oil from a reservoir is assisted by injecting a diluent into the reservoir formation to reduce the viscosity of the crude oil therein. Such techniques have been applied to the recovery of various oils, including the recovery of heavy oils and the enhanced recovery of medium and light oils. The diluent is intended to mix with the crude oil and form a mixture which has lower viscosity than that of the undiluted oil. One diluent which has frequently been proposed or used for this purpose is supercritical carbon dioxide. Another is low molecular weight liquid hydrocarbon which may be a distillation fraction such as naphtha. Mixtures of short chain alkanes, such as methane and ethane or propane and butane have also been used for this purpose, notably in the vapour extraction (VAPEX) process.
A factor which has sometimes been overlooked, but which can be relevant and even be a potential obstacle to such techniques is the possibility of asphaltene precipitation within the reservoir formation. Asphaltenes occur in varying, and sometimes quite substantial amounts in crude oils. They are a group of organic materials in which the molecules contain fused aromatic ring systems and include nitrogen, sulphur and/or oxygen heteroatoms. They are accordingly more polar than the other fractions of crude oil (saturates, aromatics and resins). They are believed, by some researchers, to occur as colloidal suspensions in crude oil and are prone to separate out if the oil is subjected to a reduction in temperature or pressure, as frequently happens during production from an oil well. Asphaltenes separate out if crude oil is mixed with a less polar diluent (notably a low-boiling n-alkane) and they are generally defined as the fraction of crude oil which is precipitated by addition of n-pentane
or n-heptane but which is soluble in toluene.
The separation of asphaltene from crude oil has been variously referred to as flocculation, precipitation or deposition. A modern view is that nano-aggregates of asphaltene molecules flocculate to form a precipitate and this may deposit on adjacent surfaces. It is a well recognised issue that asphaltene may separate from crude oil and accumulate as an undesirable deposit within production, storage and transportation equipment. Remedial treatment of wellbores and near-wellbore regions with solvent and/or heat to remove deposited asphaltene is a regular commercial operation.
It has also been recognised that asphaltene can precipitate within a formation if a viscosity reducing diluent is injected into the formation and that this can cause significant formation damage. There have been proposals to include an asphaltene solvent in a viscosity-reducing diluent which is injected into a reservoir formation to assist oil recovery. Hwang and Ortiz in “Mitigation of asphaltics deposition during CO2 flood by enhancing CO2 solvency with chemical modifiers” Organic Geochemistry vol 31, pages 1451-1462 (2000), investigated the effect of adding various solvents to supercritical carbon dioxide used to enhance oil recovery. The amount of added solvent was arbitrarily set at 10% of the carbon dioxide and solvent mixture. They demonstrated that addition of various solvents and solvent mixtures to the carbon dioxide achieved a much greater extraction of oil and with a reduced total amount of carbon dioxide required to maximise recovery. They also showed that the carbon dioxide solvent mixtures reduced the amount of asphaltene remaining in the geological formulation compared with using carbon dioxide alone. The solvents which were tried were toluene, a light aromatic hydrocarbon mixture, alcohols, and mixtures of alcohol and toluene. The aromatic materials were found to be more effective than the alcohols.
US2007/295640 proposes treating a formation with a composition containing a viscosity reducing diluent (which was a substance that would be an asphaltene precipitant if used alone) together with an asphaltene solvent. Possible precipitating diluents include light hydrocarbons as well as carbon dioxide. The compounds suggested in this document as asphaltene solvents are aromatic and substituted aromatic compounds.
Some other documents can be identified, in retrospect, as utilising mixtures in which an aromatic asphaltene solvent was present. U.S. Pat. No. 4,004,636 taught a process of treating a tar sand formation with a multiple solvent system containing both a first component which is the liquefied form of a normally gaseous material such as carbon dioxide or a short chain hydrocarbon and a second component which is a normally liquid hydrocarbon. Suggested normally liquid hydrocarbons included some such as hexane which are asphaltene precipitants and some such as toluene which are asphaltene solvents. The former category, eg hexane, was preferred on economic grounds and it was reported, expressing some surprise, that these did not cause asphaltene precipitation. The intention was that the first component of the mixture would revert to its gaseous state within the reservoir formation and drive oil from the reservoir towards a production well, while the normally liquid hydrocarbon acted as solvent. Similar disclosure is found in U.S. Pat. No. 3,954,141 and U.S. Pat. No. 4,007,785, while U.S. Pat. No. 4,071,458 and U.S. Pat. No. 4,026,358 use an aromatic solvent saturated with carbon dioxide as the diluent. U.S. Pat. No. 5,139,088 taught a process in which an aromatic fraction of the extracted oil was recirculated into the reservoir formation to act as the diluent.
In summary, the materials which have been added successfully to a viscosity-reducing diluent to act as an asphaltene solvent have all been aromatic hydrocarbons. These originate from petroleum but are somewhat expensive products of the refining process.
U.S. Pat. No. 5,117,907 taught enhanced oil recovery using supercritical carbon dioxide to which trichloroethane was added in order to increase density and viscosity of the supercritical carbon dioxide. U.S. Pat. No. 4,800,957 taught the use of alcohols or ethylene glycol as additive to supercritical carbon dioxide for a similar purpose. These documents do not mention asphaltene and do not suggest that there was any asphaltene precipitation even in the absence of the additive.